Atrops7 already gave the basics of the solution you’re looking for. This has come up before on the forum where people looking to alow site visitors to setup E-mail addresses for the domain through the website. There’s no pre-made solution I’ve heard of, but basicaly you’ll have to build some scripts that will take the info from the some kind of submit form, and post it to the panel. Your script could for instance hook up with a mysql database to get the webid info, then post the data to the panel - in the same way it would if you were doing it in the panel.
If you only want one or a few other people to be able to send out stuff, you’ll want to look into password protecting your interface. This would provide a nicer solution than making people setup a webid, and worrying about what else they might accecdintaly have access to.
So it won’t be easy, and will require a good bit of programming, but you can do it. One person contacted support about the E-mial example above, and they explained that they didn’t have any systems to help accomplish it, but were all for such a solution as I described. It’s definatly ok with them, and within TOS - but you are still responsible for the account regardless of who’s using it.
Hope this helps.